Question:
Find the principal value of each of the following :
$\cos ^{-1}\left(\frac{-1}{2}\right)$
Solution:
$\cos ^{-1}\left(\frac{-1}{2}\right)=\pi-\cos ^{-1}\left(\frac{1}{2}\right)\left[\right.$ Formula: $\left.\cos ^{-1}(-x)=-\cos ^{-1}(x)\right]$
$=\pi-\frac{\pi}{3}$
$=\frac{2 \pi}{3}$
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